Some days peanuts, some days shells

This morning I bought 16x18 and 24x24 canvases and gessoed them a warm, mid-value taupe. It's a task I enjoy - mixing the gesso and acrylic color, painting it on, then wiping it off with a towel until there's an even thin coat on the front and sides of the canvas. I went through my photos (which are in various sizes - 5x7, 4.5x6, 6x8 and 8x10) and tried to match the right image to the right canvas. It's a mostly intuitive process, and again, one I enjoy.
Then comes (insert sounds of shrieking and gibbering here) math.
I grid my photos and canvases to transfer the image. When photographs and canvases don't divide evenly I have to figure out whether losing or adding some inches will work with a given image. And that depends on the specific image and is not predictable. Hours of painful, fumbling use of a calculator and ruler later, I have three pairs of gridded photos and canvases, including an 18x24 canvas I already gessoed.
Now the drawing begins.